The answer is yes but don’t expect it to consume (dump) as much electricity as it is rated for. The lower the voltage of the electricity you want to dump (the voltage of your battery system), the less electricity the heating element will consume.

The first thing to stop worrying about is whether the element is rated for AC or DC. Both will work fine and there will be no difference in performance.

If the heating unit/element has some type of fan for circulation, thermostats, electronics or remote control devices it will not be suitable for a dump load as these add-ons will not work on DC electricity.

Now let’s look at an example:Let’s pretend your solar system is 48 volts (which is really 60 volts) and your heating element is 2000 watts at 120 volts as in the photo above. At 120 volts the amperage of this element is 16.67 amps.

At 48 volts the amperage will only be about 8.33 amps or 500 watts which is is only 1/4 of the element’s rated output.

Here’s why..

In a 48 volt system the actual voltage you will be dumping power at will be closer to 60 volts, the bulk charge voltage of your batteries.

First we need to find the resistance of this heating element. A heating element is a resistor.

OHMS=VOLTAGE/AMPS
OHMS=120/16.67OHMS=7.2

Now to calculate how much electricity (in watts) this element will consume at 48 volts (which is really 60) we use the following formula:

WATTS=VOLTAGE X VOLTAGE/OHMS
WATTS=60X60/7.2WATTS=500

At 60 volts, your 120 volt heating element will consume 500 watts (8.33 amps x 60 volts). You may have noticed this is exactly 1/4 of the power at 120 volts and you are correct.

If you were to use a 240 volt heating element in a 48 volt system, the power dumped would only be 1/16 of the element’s rated power output.

As the voltage difference gets larger between the heating element and the battery system, the amount of dumped power is less and less.

Fortunately it is very easy to calculate how much electricity will be dumped with the following charts.

Using the above chart let’s imagine we found a water heating element with the following rating:

2400 watts at 240 volts AC

Our battery bank is 24 volts so we will be dumping power at about30 volts.

How many watts will our new found water heating element be able to dump?

The answer is 37.5 watts!

Using our chart we found that to convert an element from 240 volts to 24 volts we need to divide by 64.2400 WATTS DIVIDED BY 64 = 37.5 WATTS

AC Heating Element Considerations and Tips

Water heating elements are made to be immersed in water. If you use one in the open air it will almost always burn out and be ruined. However, if you are using a 240 volt element on a 12 volt battery system it will only consume 1/256th of its rated wattage. That tiny amount of heat is not likely to damage the element even if in the open air.

Do not use any AC heater that has a built in fan. The fan will not operate at low voltage DC and could even catch fire over time.The fan is made to distribute heat in your room as well as keep the heater cool. If you disable the fan the heater might overheat causing a fire. But if you are only producing a tiny amount of heat disabling the fan will probably not cause any problems.

Polarity is not relevant. As a heating element is nothing more than a resistor it does not matter how it is wired. There is no positive or negative terminal on a resistor.

Only use elements rated for continuous usage. Although you could use a burner from an electric stove, keep in mind a burner is not really designed to operate continuously for days and days. Some good examples of dump load heating elements are water heating elements, electric baseboard heaters, electric space heaters (that have no fans).

Seriously consider using your extra electricity to heat water. Dumping excess electricity into an air heater does not make much sense. When you need the heat in the winter, it is likely you have no extra power to dump. In the summer when you have lots of extra electricity, heating the air just adds to your cooling costs.

Do not use AC heaters that have built in digital thermostats or electronics or switches of any type. All of these electronics will be damaged by DC and could possibly start a fire. Only use heaters that have an on-off switch or nothing at all. Electric baseboard heaters generally have no extra controls making them suitable.

BONUS QUIZ:

There are four different heating elements shown in the photo near the top of this page.

1500 watt 120 volt baseboard heater

1800 watt 240 volt water heating element

2000 watt 120 volt water heating element

2200 watt 240 volt space heater

If you had all four of these heating units in your basement, which would be the best for your 48 volt, off the grid electrical system dump load?

My first choice would be the 3rd one, the 2000 watt 120 volt water heater.

First, it is usually preferable to make hot water with our extra power instead of hot air.

Second, the #3 element in the list will be able to dump up to 500 watts (2000 divided by 4) which is more than any other the others.

#1 will dump 375 watts

#2 will only dump 113 watts

#4 will only dump 138 watts

It is almost always best to use a 120 volt heating element instead of a 240 volt heating element as the power output at 120 volts will be multiplied by 4 compared to the same wattage 240 volt element.

For example:
A 1500 watt 120 volt heater will consume 375 watts while a 240 volt element will consume only 94 watts at 48 volts. This is a factor of four.

So looking at the math, I can use the AC heating element in the water heater on my sailboat as a dump load for a wind generator. But do I have to disconnect the DC wiring from the element if we are connected to AC shore power using the water heater? Or can we leave the wiring be? I would think the AC current would try to alternate on the DC system and maybe cause some damage or a fire, but maybe not.

The short answer is a big NO you cannot connect a DC dump line to the same heating element that could be powered by AC shore power.

If you decide to disconnect the DC dump line every time you plug in the shore power your wind turbine will no longer have a dump load. Of course you could disconnect the DC dump line from the water heating element and move it over to another dump load but in my experience people always forget to do things like this. Or someone else thinking they’re helping plugs in the AC shore power without realizing they have DC connected to the same circuit.

The second you plug in to the AC shore power, the AC electricity will backfeed into your charge/dump controller. It will be destroyed and has a high potential for fire. That should be the extent of the damage.

I have seen folks add either a mechanical relay or a solid-state relay to disconnect the DC dump line when the shore power is plugged in. Solid-state relays seem to be fast enough to do this without damaging the dump controller. However now there is no dump load on your wind turbine. Now you have to create a secondary dump a load for your wind turbine. You can see how this could get complicated fast.

The best option is to have 2 elements in your tank, one for AC and one for DC (regardless if you are using AC elements or DC elements). Most hot water heating tanks have a whole near the bottom for an element and a whole closer to the top for another element.

If you rely on the DC dump load as your main source of hot water, put the DC dump load at the bottom and the AC shore power fed element at the top.

If you rely on AC sure power as your main source of hot water, put the AC element at the bottom and the DC dump load element at the top.

If your hot water heater only has one threaded (or square 4 bolt) hole for one element then get in touch with me again and I will give you some options.

Hope this message finds you well,
Thanks again for your great question,

Hi there , firstly thank you for such great info on dump loads.
I am battling to find a 48v DC element and am hoping using a 220v AC element can work.
I have a hydro turbine (powerspout) on a 48v battery bank. The powerspout can produce an output of 1200 w .
Would I require the full wattage of this output as a dump load? , and hence a hell of a lot of 4 kW 220v AC elements?
I will be installing an SSR on the aux output of the Mppt controller

I am so sorry for the delay. I have had two flus back to back and have been barely able to keep up with shipping orders. My question is can you get 120 volt water heating elements? You would only need two 2000 watt 120 volt elements to make up a 1000 watt dump load as 120 volt elements use 1/4 the power at 48 volts. If not, let me know and I could probably get you one 900 watt 48 volt heating element that I have in stock for about $129 CAD. You can always always turn the water spout down to 900-1000 watts or so as 1200 watts is a lot of power or you could use the one 900 watt element for heating water and have a back up air heater to take care of the rest of the power. Remember you will always be using some of the water spout’s power to operate your home or cottage and your batteries will always need some power just to maintain themselves so you will not really need the full 1200 watts in dump loads. Hydro is nice as you can simply close a valve a little until you get your power consumption down to what you need. Make sure you use a large chunk of aluminum to keep your SSR cool or get a heat sink from a solar solar store like us or somewhere local. I remember my first SRR catching fire in Haiti as I did not have a large enough heat sink to keep it cool in a climate that rarely ever cools down. I hope I have answered some of your questions and sorry for being so late…Jody

You are absolutely correct. If you have a 1000 watt dump load it will dump 1000 watts. If you add another in parallel you will be able to dump 2000 watts. You can put as many in parallel as you need to for your application. Similar to lighting your room with one 100 watt light bulb and adding a second. Now you are consuming 200 watts when you turn the light switch on. Take care…Jody

I have a 36 liter water heater in front of my Noritz natural gas tank-less water heater.
I bought the 36 liter electric as a backup incase the gas unit fails.
The 36 liter is a 1500 watt at 120 volts AC
I understand that I can not use the AC controls with DC but could I just run my 250 watt Kyrocera panel straight to the element? I doubt that 1×250 watt panel will ever cause an overheat or could it?
This is a 60 cell panel btw.

I am wondering whether I should dump/divert from my battery bank or directly from my solar panels.

I am planning to have 4 panels (345W each at 57V) wired in 2 banks of 2 panels and thus bringing a peak 114 volts at 12 amps into my charge controller. I will be using a solid state relay controlled from a Outback FM80 charge controller. The 120V water heater that I am planning to acquire has a 1500 watt heating element.

If I dump from the 24V battery bank , it sounds like I will consume 300 watts. If I wire the relay to direct DC voltage straight off of the PV array input then I think it should peak at heating at 1425 watts which is near the full watt rating but means bringing some heavy gauge wire and higher voltage into my hot water heater location.

In this installation, I will also be using a solar hot water collector and thus the electric water heating is a bonus intended to make things happen faster.

Based on prior comments, it sounds like if I dump from the battery bank then I should not run the relay in PWM mode due to hardship on the inverter.

Thank you so much for your question. I would definitely dump the power using PWM and a solid state relay directly from your battery bank and not your solar array. The battery bank will provide a steady voltage (24-30 volts) to your dump load. If you connect your dump load between the solar array and charge controller you will have nothing but problems.

First we do not know what voltage will be produced when the solar array is loaded down by the 1500 watt heating element. It will not be as high as you might think. You could always try connecting your array to the heating element when you get the array installed to see where the voltage settles and how much power will actually be dumped.

Second and probably the biggest problem is how the FM80 is going to react to having a load on the input. When the batteries are charging, everything will be fine. However when the batteries are starting to fill up the FM80’s relay driver will begin pulsing to allow small amounts of power to go into the dump load which will drop the input voltage of FM80. Now the FM80 will have to process an input voltage that is changing many times per second. This would be catastrophic to the FM80. It is designed to take a fairly stable voltage and reduce it to charge your battery bank. It is not designed to have solar power that is being pulse width modulated with the voltage changing hundreds of times per second.

To answer your last sentence….dumping from your battery bank does not have any impact on your inverter as you are not dumping through your inverter. You would connect your dump load directly to the battery bank (with over-current protection of course). When you dump 120 or 240 volts directly from your inverter, you cannot use PWM as your inverter would be forced to provide power on and off many times per second. No inverter can handle this type of abuse.

My recommendation is to buy a 24 (30) volt water heating element that is good for up to 1400 watts. We have a few here so they are available. Also get a 60 amp solid state relay good for at least 60 volts with a 12 volt trigger (it is turned on by applying 12 volts). We used to sell them here for $50 or $60. Also get a large aluminum heat sink like you would see on a desktop computer’s processor. You can also use a thick and large piece of aluminum. Solid state relays create a lot of heat and will catch fire if not kept cool. I installed a 60 amp SSR turn a pump on and off. The pump only used about 30 amps so I wasn’t concerned about overheating the SSR. Within 24 hours it caught fire. The second time I installed 2 SSRs in parallel and mounted them to a piece of aluminum. That worked perfectly.

I hope I haven’t been too confusing. If I can help please let me know…

Hi .great info .
Just wondering if the power output from a solar panel can be sent straight to a 240V heating element seeing it is so small output in system also seeing it will be winter time it will be even less output.
Can I do the same with the new F Wave 319V flexable sol panel.
And also and also power straight from wind turbine that has wild AC power sometimes 3phase power coming out and sometimes 1 phase depending on the wind speedway

We have a small, off grid 12vdc solar system (660 watts of PV) and are about to hook up to a community solar micro-grid. I’m going to keep our roof top solar/battery system as back up in case the grid fails and also would like to use it to heat hot water as a ‘full time diversion load’. From your article it looks like I would have to purchase a lot of hot water heaters to use up even our small supply of power. Does it make sense to simply power an electric hot water heater at 120vac using our existing inverter and batteries?

I would be more likely to buy a 12 volt water heating element for about 50-100 dollars and control it with a 12 volt relay driver from your inverter or charge controller and also with a solid state relay. If you decide to use a 120 volt element and your inverter make sure you use an on-off relay and not a solid state relay that is pulse width modulated. Pulse width modulation will be very hard on your inverter. Sorry you cannot find a link to the comments. I will try to fix that…Jody

I assume I need an element that can handle the full load or more of my panels, is this correct?

Currently I have about 140w of solar charging a 12v battery. I have a mix of panels, the biggest one is 12v 100w, the other 40 watts is made up from a few different 12v panels.
I do not know the specs of the panels more then that. Hopefully that’s enough information.
Cheers

You will only need an element that can handle 100-150 watts. If you have 140 watts of solar, the most they will ever produce is about 100-120 watts depending on the charge controller type (mppt or pwm), the age and capacity of your batteries, the age of your modules, and the fact they will only produce close to their max output when the sun is at 90 degrees to the solar modules. There will also always be a little power going into the batteries to float charge them. If you had an element that could handle 120 watts or a little more you will be in good shape. Are you going to use a diversion controller to dump the power as the batteries reach a full state of charge? Thanks for your question and have a great day…Jody

We are looking at using the dc load in a heater but do not know how to control the temperature so the fish do not die? Do you have a DC thermostat or other recommendation for both battery DC and direct DC solar?

What I would do is actually use an AC thermostat in conjunction with a DC solid state relay and AC or DC element. What would happen is you would put a tiny bit of low voltage DC likely 6 volts or 12 volts through the thermostat to control a DC solid state relay (SSR). The SSRs I use here for all sorts of projects operate on just a few milliamps which do not harm the ac thermostats at all. Go to for a crude drawing. The solid state relay carries all the power while it only takes a few milliamps to trigger the SSR which any AC thermostat will handle just fine. I left out fuses/breakers in the drawing but they are a necessity. Hope that helps or if you have any questions please let me know. You will be able to find SSRs on eBay quite easily. I had 500 custom made for me a few years ago that are rated for 220 volts DC at 60 amps. The power required to turn the relay on is 3-32 volts at 3 or 4 milliamps. They cost $49 from me. No matter what SSR you use be sure to mount it to a massive piece of aluminum or some kind of heat sink or it will burn out or catch fire. They make a fair amount of heat. Take care…Jody

Hello, I was wondering if you could help me with a question that I have been trying to figure out and am just a little confused. I have SunPower 435 watt 72 volt panels. What heating element do you suggest that I use for a preheat tank. I am new to solar and want to make my system as efficient as possible. Thank you in advance.

Hi,
Thanks for your question. Could you tell me a little more about your system? How many solar modules do you have? Are you just using them to connect directly to a water heater element to heat water or is this a dump load as part of a bigger off grid or on grid power system? Thanks…Jody

LOOKS LIKE THE PANELS WILL GIVE YOU ABOUT 6AMPS. A STANDARD 3000 WATT HEATER (240) WILL WORK FINE. IF YOU HAVE TWO PANELS USE TWO ELEMENTS CONNECTING ONE PANEL TO EACH OF THE ELEMENTS…YOU WILL THEN GET CLOSE TO 1KW OF POWER THAT ACTUALLY WILL BE USEFUL. REMEMBER TO DISCONNECTED ANY AC CONTROLS IN THE HEATER OR SWITCHS BETWEEN THE PANELS AND THE ELEMENT…THEY WILL NOT WORK AND ARE A FIRE HAZARD!!!
DAVE

Hi Dave,
Thank you for your help. Let’s look at your suggestion and see what we get.

Your first comment was the module will produce about 6 amps.

WATTS/VOLTAGE=AMPERAGE
435/72=6.04

However that 6.04 amps is at 72 volts. There are a few issues with that. The 72 volts is likely the nominal voltage and the actual maximum power point voltage of the module is likely higher but we don’t have that information. Second we do not have 72 volt heating elements off the shelf.

You suggest we connect one panel to one 3000 watt 240 volt element and all will be well. Let’s do the math and see what we come up with.

First we need to find the resistance of your recommended heating element by using the formula…
OHMS=VOLTAGE/AMPS
OHMS=3000/240
OHMS=12.5

For every one of your solar modules you connect directly to a 3000 watt 240 volt heating element you will get 414 watts out of it. The peak will actually be higher as the maximum power point voltage is likely closer to 102 volts making the solar module put out it’s full rated power for the few minutes a day the sun is perfectly perpendicular to the sun.

You could use up to 6 or 7 solar modules (I would go with 6 as I would rather not stress the water heater element and make it last longer) wired in parallel to each 3000 watt 240 volt element, rather than one element per solar module as it gets tricky to find a hot water heater with several element holes.

DAVE IS ABSOLUTELY CORRECT. ONLY USE THE ELEMENTS. DO NOT USE AC THERMOSTATS, SWITCHES OR ANY AC CONTROLS. THEY WILL BE DESTROYED IF YOU ARE LUCKY AND CATCH FIRE IF YOU ARE NOT. YOU WILL NEED UL LISTED DC BREAKERS OR FUSES INSIDE UL LISTED DC FUSE OR BREAKER PANELS TO PROTECT THE WIRING FROM ARC FAULT OR SHORT CIRCUIT.

We have tons to those products and can airmail them to you as necessary. You could use your off the shelf thermostat with a DC solid state relay if you don’t want your preheater to boil. I can help help you with that too if needed.

Jody
I see you correctly stated ohms law, but error-ed in your implementation of it. The correct answer is 19.2 ohms. Knowing that, (and using ohms law) one can calculate that at 78 volts the element will dissipate around 270 watt …a pretty good match for the panels(I am guessing they are 250-275w panels). Since most heaters have two elements,using two elements matched and each connected to a panel would be a nice easy system. yes, you could parallel them (increasing wire size), but once you have personally experience the negative performance created by shadow effects, you will do anything and everything you can to decouple them!!

I am interested in what dc components you have available for sale..let me know…thank you
Dave

For every one of your solar modules you connect directly to a 3000 watt 240 volt heating element you will get 270 watts out of it. The peak will actually be higher as the maximum power point voltage is likely closer to 102 volts making the solar module put out it’s full rated power for the few minutes a day the sun is perfectly perpendicular to the sun.

As we are told the solar modules are 435 watt I would definitely NOT use a 3000 watt 240 volt element as we would only be using a little over half of the solar module’s output. I will have to think about this and post my recommendations later today or tomorrow.

When you wire solar modules in parallel, a shadow on one will have no effect on the other. You could have 4 solar modules in parallel and cover three of them. The fourth module will still operate as normal.

If you wire them in series you are absolutely correct about the shadowing effect. Drop a leaf on one solar module and the whole array will go down.

Edward I would still love the answers to the questions I originally asked:

Could you tell me a little more about your system? How many solar modules do you have? Are you just using them to connect directly to a water heater element to heat water or is this a dump load as part of a bigger off grid or on grid power system? Thanks…Jody

Dave I have every DC fuse and circuit breaker known to man. We have the larger frame 100, 175 and 250 amp breakers, the Midnite Solar MNEPV breakers, Midnite’s MNEDC breakers, Class T Fuses in 110, 200, and 300 amps, 60 amp DC solid state relay’s that are good for up to 220 volts DC, Midnite Solar’s shunts, busbars, many aluminum and copper lugs for wire from 14g to 4/0 (0000) and lots of other stuff. If you are looking for something weird we probably have it.

Since I work from home I can just air mail packages. I can accept PayPal, Visa and MasterCard and money orders.

Please correct any other mistakes you see. I can’t believe how badly I messed up ohm’s law.

Hi. Can I connect four solar panels IN SERIES to supply electricity to a 120 VAC water heater? The panels I already have are Canadian Solar model CS6P-230P, with an open circuit voltage of 36,8 VDC and an optimum operating voltage of 29,6 VDC. In net, I would be supplying around 120 VDC instead of 120 VAC.
Thanks, Lino

You can absolutely do this. With the price of photovoltaic modules becoming so inexpensive this is going to become the norm. No plumbing, no pumping, no freezing and no controls to fail. Your MPP voltage of the 4 modules in series is perfect. You will be very pleased with the results. Just be positive the water heating element is 120 volts and not 240 volts. At 240 the element would only make 1/4 the hot water the 120 volt element will.

Another issue, DO NOT use 120 volt AC switches, breakers, fuses or thermostats. You WILL end up with a fire if you do. If you are unsure what to do contact me at my email below. There are correct ways to do it, you just need everything rated for high voltage DC.

The above examples are very confusing because you don’t use the exact values as given by the proper equations. The answer is a good estimation but very unreliable as far as an example and exact calculations. It is hard to understand where the 60V comes from without the author showing exactly how he estimated 60V.

Dump loads are always confusing. The 60 volts is used as the voltage on a 48 volt system as 60 volts is an average dump voltage. In a 48 volt system you would not be dumping any electricity until the voltage gets high (58-62 volts) so we use 60 as the average voltage the dump load will be operating. We do not have to calculate down to the nearest 100th of a volt as it is not necessary. Dumping voltages change constantly due to the amount of electricity being dumped, the ambient temperature, the age and quality of the battery bank, the temperature of the heating element and several other factors. We are not operating a space shuttle, we are simply dumping electricity into a dump load. It is very much the same as the variation in the “off the shelf” heating element.

The same 240 volt element may operate at any where from 208 volts to 250 volts depending on the voltage the utility is supplying. A 2000 watt heating element may consume anywhere from 1502 watts (at 208 volts) to 2170 watts (at 250 volts). That is a large variation so the manufacturer has to pick a voltage and wattage for their label. We do the same, we pick a voltage and go with it!

good day! i was wondering if i can use a clothes iron for a dump load? i have an iron rated at 1100 watts at 120 volt ac, im currently running a 12 volt system,but my new addition is a treadmill windturbine i made and it sometimes pushing out 50 + volts dc. can u tell me if the iron can work?thanks!

Hi Anthony,
Thanks for the good question. The heating element of the clothes iron will make a great dump load. The only issue with using the iron is that the on/off switch and the thermostat inside the iron are rated for AC instead of DC. It is likely they will be damaged (and possibly catch fire) very fast if you simply hook the 12 volts DC to the power cord of the iron. It will be able to dump about 17 watts at 12 volts which is not very much.
The treadmill motor’s voltage output will be controlled by the voltage of your battery bank. Even though the motor will output 50 volts open circuit, the second you connect it to the battery bank, it will become a 12 volt wind turbine. If you were to connect the same motor to a 24 volt battery bank, it will now become a 24 volt wind turbine. The battery bank will become the deciding factor of the wind turbine’s voltage.
It is similar to a car alternator. We use a standard Ford alternator here connected to a 12hp diesel engine to charge our bank when the sun is not shining. Even though the alternator is rated at 12 volts it will make whatever voltage you want depending on how fast you spin it and what the voltage of your battery bank is. Our system is a 48 volt system and the Ford alternator has no trouble charging 60 amps at 48-60 volts (2880-3600 watts). It obviously has no regulator. If I were to connect it to a 12 volt battery bank, the output would be 60 amps at 12-15 volts (720-900 watts). The amperage stays the same (as it is limited to what the alternator can produce) but the voltage is only limited by how fast you spin the alternator (and the voltage of your battery bank).
Good luck with your wind turbine and dump load….

I had no idea they made 435 watt panels..I just am a cheapy looking at 50 to 90 cent per watt panels. These panels deserve a 4500 watt element in each hole, and in parallel if you want, then they could also to heat the house!
Do you have a price list of your dc parts (small quantities)
Thanks
dave